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Old 6th Dec 2016, 3:07 pm   #1
Nickthedentist
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Default Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Hello everyone,

I'm trying to restore a rather unloved Roberts P5A from 1948, like this one: http://www.radiomuseum.org/r/roberts_unknown.html

It's a mains-only portable with an isolating transfomer supplying the five International Octal valves' heaters but with HT derived directly from the mains, making it one of that rare breed: an AC-only, live-chassis set (complete with non-isolated gram input!).

Apart from cosmetic problems, the main trouble seems to be that the KT61 audio output tetrode has lost its vacuum, and I'm not prepared to spend the best part of £100 (or anything like that TBH) on a replacement: http://www.r-type.org/exhib/aad0232.htm

Forgive my naivety, but is it possible to use a pentode here instead? If so, could I press into service something as easily obtainable as an EL84 as an interim replacement? http://www.r-type.org/exhib/aaa0028.htm

Obviously, I don't want to spoil the set by changing bases, but would be happy to make-up an adaptor using the dead valve's base and make minor wiring changes on the chassis etc.

Many thanks,

Nick.
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Old 6th Dec 2016, 3:36 pm   #2
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

I've found a used KT61 which I'd need to test. Make me an offer if you like. Proceeds to Ipswich Branch of the RSPCA.
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Old 6th Dec 2016, 3:43 pm   #3
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Thanks, Graham, that's kind, I will bear that mind.

I'll have to poke around in my box of unsorted pulls too as there might be something more suitable under my nose.

N.
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Old 6th Dec 2016, 4:27 pm   #4
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

A 6V6 should be OK. You just need to check the current and increase the cathode resistor if necessary.
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Old 6th Dec 2016, 5:20 pm   #5
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Thanks Paul, I may even have a 6V6GT lying around, and new manufacture "equivalents" with a reasonable reputation are available for sensible prices e.g. https://watfordvalves.com/product_detail.asp?id=1343

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Old 6th Dec 2016, 5:39 pm   #6
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Most 6.3V octal audio output tetrodes and pentodes have the same pinout and are to some extent interchangeable.

You could certainly also build an adaptor from the old KT61 base. An EL41 or EL84 should be fine subject to the usual caveats, as should many others even including the pentode section of an ECL80. The OP transformer turns ratio may not be ideal and the valve slope may be different, but this doesn't seem to matter much in practice.
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Old 6th Dec 2016, 6:47 pm   #7
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

I tested the used KT61.

Anode current at specified grid voltage 24mA (spec 40mA).

Grid voltage to give 40mA anode current 2.2V (spec 4.4V).

gm 9mA/V (spec 10.5mA/V)

It'll probably work OK in a set.
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Old 7th Dec 2016, 10:57 am   #8
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Thanks for all the replies.

Graham, that's very kind of you, but I braved the lock-up last night with my head torch, and found a huge box of valves that a mate gave me while I was at primary school over 30 years ago; he knew I collected old junk and had pulled them out of stuff at a tip while on holiday in the Lake District. Amazingly, the contents of the box was undamaged in spite of 3 house moves and I found several examples of potentially suitable substitutes e.g. 6P25 and 6V6G.

If they're faulty, I will get back in touch with you, but realistically, I may be sorted.

This is definitely one of those occasions where I'm glad I resisted pressure from everyone around me to "de-clutter"

Nick.
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Old 7th Dec 2016, 11:14 am   #9
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Quote:
Originally Posted by Nickthedentist View Post
.....is it possible to use a pentode here instead?
Hi Nick

I see from your first post snip of the circuit that you must have a service sheet. Does it list the output valve as a KT61? The radiomuseum link you provide lists the output valve as an EL33 (which of course is a pentode).
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Old 7th Dec 2016, 11:56 am   #10
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

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Originally Posted by Nickthedentist View Post
Graham, that's very kind of you, but I braved the lock-up last night with my head torch, and found a huge box of valves that a mate gave me while I was at primary school over 30 years ago; he knew I collected old junk and had pulled them out of stuff at a tip while on holiday in the Lake District. Amazingly, the contents of the box was undamaged in spite of 3 house moves and I found several examples of potentially suitable substitutes e.g. 6P25 and 6V6G.
Nick.
No problem Nick. At least I now know that the KT61 has some life left in it.
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Old 7th Dec 2016, 12:24 pm   #11
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

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Originally Posted by ukcol View Post
Does it list the output valve as a KT61? The radiomuseum link you provide lists the output valve as an EL33 (which of course is a pentode).
...er, I'm glad someone is awake, Colin!

You're absolutely right, the Trader Sheet (826) does indeed list it as an EL33, and it's quite rightly described as a "pentode output valve" in the circuit description. Presumably the dead KT61 has just been shoved into the valveholder, as I can't see any evidence of it having been re-wired.

So, I'll have to rummage around in the Big Box again tonight. Apologies to everyone I misled (especially Graham).

Would an adaptor using the base of the old valve and an EL84 be an option?

N.
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Old 7th Dec 2016, 12:42 pm   #12
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

No rewiring necessary. The EL33, KT61 and 6V6 pinouts are all the same.

An EL84 in an adaptor would be fine, but it seems a bit pointless when you already have a plug in substitute.
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Old 7th Dec 2016, 12:42 pm   #13
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Some sources list the EL33 and KT61 as being equivalents even though one is a pentode and the other a beam tetrode.

That might explain why a KT61 was fitted?
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Old 7th Dec 2016, 12:57 pm   #14
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Thanks chaps.
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Old 7th Dec 2016, 1:00 pm   #15
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Yes quite a few listings describe the EL33 and KT61 as if they were the same thing and no doubt in this application it would be difficult to detect any difference in performance.

The EL33 seems to attract quite high prices too, although perhaps not as silly as the KT61.
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Old 8th Dec 2016, 1:05 am   #16
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Obviously the EL audio multiplier isn't as high as the KT one

Certainly they're less kinky
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Old 9th Dec 2016, 8:45 am   #17
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Quote:
Originally Posted by paulsherwin View Post
A 6V6 should be OK. You just need to check the current and increase the cathode resistor if necessary.
Well, I fitted a 6V6G from the junk box and increased R17 from 160R to 560R which brought the anode current down from a shade over 40mA to the 29mA specified in the Trader Sheet data (calculated by measuring the voltage drop across R18). Have I done that right?

Results are good but there is perhaps a little more distortion on the audio than I would have expected. I will have to experiment to see whether it's coming from the radio or AF amp sections, or even the Droitwich transmitter (Radio 4 LW).

All waxies have been replaced and the voltage on the output valve's control grid is essentially zero.

N.
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Old 9th Dec 2016, 9:36 am   #18
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

I make that over 16 volts -ve grid bias, perhaps too much, if the rectifier/transformer(s) can take it I would reduce the grid bias, try somewhere around 10 volts -ve.

Typical bias voltages for Class A:

http://www.r-type.org/pdfs/6v6g.pdf

Lawrence.

Last edited by ms660; 9th Dec 2016 at 9:51 am.
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Old 9th Dec 2016, 12:07 pm   #19
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

The 6V6 itself may be a bit tired, assuming it's a pull. You could experiment with any other output valves you have with the same pinout.
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Old 9th Dec 2016, 4:25 pm   #20
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Default Re: Substitution of a KT61 in a 1948 Roberts P5A with something cheaper?

Thank you both.

Paul, I seem to have loads of other suitable valves in the box which I could try. They're all untested pulls though.

Quote:
Originally Posted by pwdrive View Post
I make that over 16 volts -ve grid bias, perhaps too much, if the rectifier/transformer(s) can take it I would reduce the grid bias, try somewhere around 10 volts -ve.
This is all new stuff to me so apologies if I'm asking daft questions. Are the following thoughts correct?

Assuming the control grid is held at 0v, to reduce the amount of -ve grid bias I presume I'd need to increase the voltage on the cathode by increasing the value of the cathode resistor (R17) still further. This would also reduce the anode current below the 29mA where it stands currently.

But if that's the case, why do you say "...if the the rectifier/transformer(s) can take it..."?

Nick
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